1. Field of the Invention
This invention relates to a display device, and more particularly to a thin film transistor substrate of a poly-silicon liquid crystal display device and a simplified method of fabricating the same.
2. Discussion of the Related Art
Generally, a liquid crystal display (LCD) device, which includes a plurality of liquid crystal cells in a matrix configuration in a liquid crystal display panel, displays images by controlling the transmittance of light in accordance with video signals. In each liquid crystal cell, a thin film transistor (TFT) is used as a switching device to independently supply a video signal. An active layer of such a TFT is generally formed of either amorphous silicon or polycrystalline silicon (poly-silicon). Because the carrier mobility of poly-silicon is approximately hundred times faster than the carrier mobility of amorphous silicon, high-speed driving circuits can be integrally formed in the LCD panel with the poly-silicon technology.
A poly-silicon LCD device generally includes a TFT substrate provided with driving circuits and a color filter substrate provided with a color filter, with liquid crystal provided therebetween.
FIG. 1 is a plan view illustrating a portion of a TFT substrate in a poly-silicon LCD device according to the related art, and FIG. 2 is a cross-sectional view of the TFT substrate taken along the line I-I′ in FIG. 1.
Referring to FIGS. 1 and 2, the TFT substrate includes a thin film transistor (TFT) 30 connected to a gate line 2 and a data line 4, and a pixel electrode 22 connected to the TFT 30. Although either an NMOS-TFT or PMOS-TFT can be used for the TFT 30, the TFT 30 employing an NMOS-TFT will now be described.
The TFT 30 has a gate electrode 6 connected to the gate line 2, a source electrode connected to the data line 4, and a drain electrode 10 connected to the pixel electrode 22 via a pixel contact hole 20 passing through a protective film 18. The gate electrode 6 overlaps a channel area 14C of an active layer 14 provided on a buffer film 12 with a gate insulating film 16 therebetween. The source electrode and the drain electrode 10 are formed in such a manner to be insulated from the gate electrode 6 with an interlayer insulating film 26 therebetween. Further, the source electrode and the drain electrode 10 are connected to a source area 14S and a drain area 14D of the active layer 14 doped with an n+ impurity, respectively, via a source contact hole 24S and a drain contact hole 24D passing through the interlayer insulating film 26 and the gate insulating film 16.
The TFT substrate of the related art has disadvantages in that the manufacturing process is complicated and the manufacturing cost is high.